Fiber to the Antenna (FTTA) Revolutionizes Cell Site Builds

By John Celentano, TESSCO Technologies

Split-mount radio technology consisting of remote radio units (RRUs) and baseband units (BBUs) is quietly (or maybe not so quietly in some quarters) revolutionizing the way 4G cell sites are constructed, operated and maintained. RRUs are installed up on the tower next to the antennas and provide the amplification for radio frequency (RF) transmission and reception to and from the site. RRUs currently operate at licensed cellular frequencies (850/1900 MHz, 700 MHz, AWS, 2.5 GHz).

BBUs are installed at ground level in a hut or cabinet. BBUs house the circuitry for one or more air interfaces (GSM/HSPA+, CDMA/EV-DO, WiMAX, LTE). BBUs interface with RRUs and provide the modulation functions for all the over-the-air traffic between the site and mobile devices within the site's operating area. BBUs also provide the connections to the backhaul transport facilities between the site and the carrier's central switching location.

Here's the revolutionary part: BBUs communicate with RRUs via Internet Protocol (IP) signaling over fiber optic cable. RF transmission only takes place between the RRU and the antenna over a short coaxial cable jumper. This configuration is a vast departure from the classic radio transceiver at the base of the tower connected to the antennas over long coaxial cable runs that carry RF signals to the antenna from the base of the tower. Today, it is not unusual to see towers virtually encased by multiple coaxial cable runs installed on the outside of the tower.

The advantages of split-mount technology over monolithic radio transceivers include:

• The long coax cable runs are eliminated – RRUs are close to the antennas they serve so only a short coax jumper cable is needed between the RRU and antenna, thereby significantly reducing RF losses.
• Fiber cable weighs less than coax cable for the same length of run, so tower loading is reduced.
• BBUs take up less space, power, and air conditioning in the shelter or cabinet.
• RRUs are air-cooled and do not require air conditioning to operate.

Split-mount radios introduce other not-so-trivial challenges once the active radio electronics are located up the tower close to the antennas. First, fiber cable and the connectors at the end of the fibers must be handled differently, and with more care, than rugged coaxial cable. Secondly, RRUs draw DC power that must be fed from the power system installed in a hut or cabinet at the base of the tower.

Certainly, fiber cable is designed with outer coating and sheathing that allow it to operate in harsh outdoor environments. The issue is handling and protecting the all-important connectors at the end of the fiber strands, especially during the installation phase. All fiber connections are much more susceptible to dirt, even dust, and potential damage than coax. Moreover, different manufacturers' RRUs require different fiber connector styles. Once the required length of cable is determined, then connectors are added to the fiber cable, usually in the cable factory.

The other issue is feeding DC power to the RRU. And each RRU has different power requirements, again depending on the manufacturer, but in the order of several hundred watts at either -24 or -48 VDC. Power cables of 6 or 8 GA or stranded wire are used to deliver DC power over several hundred feet to the RRU.

So the question is: What is the best cabling solution for connecting the BBU to RRUs?

There are three available methods.

The first is referred to as the home run. This method involves separate fiber and power cable runs to each RRU. The cables are usually installed in some sort of weatherproof duct that is installed inside a monopole or on the outside of other types of towers. Each fiber cable is precut to the appropriate length to reach the RRU; fiber connectors are added at the factory or in the warehouse.

Another method that is finding much use in recent network builds is a composite or hybrid cable that consists of individual coated copper and fiber cables in a common, weatherproof, steel-jacketed sheath. Hybrid cables come in different configurations to feed 1, 3, or 4 RRUs at a time. Depending on the site, hybrid cables can be more than 300 feet long. Hybrid fiber/copper solutions reduce the need for separate cable installations for each RRU. As with home run solutions, the cable lengths must be precut for the installation and appropriate connectors must be added to the fiber cable at the factory. Different types of connectors are required depending on the RRU manufacturer. At the RRU end, the individual cables for each RRU fan out from a weather seal at the top of the cable in a Medusa-like fashion.

The main drawback with the hybrid cable solution is the precision required for the horizontal lengths of cable to each RRU. (See Figure 1) In addition, the weight of the hybrid cable, typically 2 pounds per foot, means that hoisting and securing the cable must be performed carefully. More important, only a limited number of RRUs can be served from one hybrid cable. This means that a full-capacity installation will require multiple hybrid cables to handle the total number of RRUs.

The third method is what we call a terminated feeder solution. (See Figure 2) Fiber and power feeders are installed separately. Each feeder cable, however, has enough capacity to handle multiple RRUs. The difference is that the feeder cable terminates in a distribution box that is mounted at the height of the RRUs. Jumpers are used to connect the terminal boxes to the RRUs. The fiber feeder consists of multiple fibers in a weatherproof sheath with the fiber count determined by the port size of the terminal, either 6 or 12 ports, and whether 2- or 4-fiber RRUs are being fed. The maximum configuration is a 48-strand fiber cable that feeds a 12-port terminal. The capacity of the terminal depends on the growth plan for the site. Precut fiber jumpers run between the terminal box and the RRUs.

In parallel, a copper feeder cable terminates in a small power terminal box at the same height as the RRUs. The power terminal box can be configured with 4 or 6 ports at the end of the feeder that consists of multiple 6 or 8 GA, or stranded copper cable. Precut power jumpers of 10 or 12 GA copper are run from a port on the terminal box to each RRU. Additional terminated feeders can be run as the RRU total exceeds the capacity of the terminals. The good news is that the overall number of vertical cables running up the tower will be vastly reduced. Plus initial and expansion installations will be much more streamlined.

In the end, split-mount radio technology has changed the deployment of wireless systems. TESSCO offers a full range of FTTA solutions for any site configuration and any radio manufacturer, and our engineers can discuss your particular installation.